In a 20, you'll have room for 1-4 fish in my opinion. All depends on which ones you get. I'd not do damsels personally, too aggressive and will limit what else you can keep. I'd not do the mandarian either, although they are small they need a larger mature tank to supply them with enough pods. Assessors, clowns, cardinals, gobies, dartfish would all make good candidates. So would shrimp and other inverts. Some might go a whole other route and do a species tank with either one scorpionfish or a seahorse/pipefish tank. Depends on what appeals to you.